Progressive web application
While web applications have been available for mobile devices from the start, they have generally been slower, have had fewer features, and have been less used than native apps. PWA features narrow the gap between user experience in web-based and native applications. Features include
- Ability to run offline
- High performance
- Background processing in service workers in a separate thread
- Access to the phone's sensors
- Support for push notifications
- An icon on the phone‘s home screen
Since a progressive web app is a type of webpage or website known as a web application, they do not require separate bundling or distribution. In particular, there is no requirement for developers or users to install the web apps via digital distribution systems like Apple App Store or Google Play. However, for many PWA providers the lack of visibility of the PWA in those distribution systems is a disadvantage compared to native apps. To varying degrees, the major app stores started to embrace PWAs, allowing them to be found in app stores.
As of April 2018, the PWA features are supported to varying degrees by the Mozilla Firefox, Google Chrome, Apple Safari, and Microsoft Edge browsers, but more browsers may support the features needed in the future. Several businesses highlight significant improvements in a wide variety of key performance indicators after PWA implementation, like increased time spent on page, conversions, or revenue.
At the launch of the iPhone in 2007, Steve Jobs announced that web apps, developed in HTML5 using AJAX architecture, would be the standard format for iPhone apps. No software development kit (SDK) was required, and the apps would be fully integrated into the device through the Safari browser engine. This model was later switched for the App Store, as a means of preventing jailbreakers and of appeasing frustrated developers. In October 2007 Jobs announced that an SDK would be launched the following year. As a result, although Apple continued to support webapps, the vast majority of iOS applications shifted towards the App Store.
Firefox released Firefox OS in 2013. It was intended to be an open-source operating system for running webapps as native apps on mobile devices, with Gaia built as its HTML5 interface. The development of Firefox OS ended in 2016.
In 2015, designer Frances Berriman and Google Chrome engineer Alex Russell coined the term "progressive web apps" to describe apps taking advantage of new features supported by modern browsers, including service workers and web app manifests, that let users upgrade web apps to progressive web applications in their native operating system (OS). Google then put significant efforts into promoting PWA development for Android. With Apple's introduction of service worker support for Safari in 2017, PWAs were now supported on the two most commonly-used mobile operating systems, Android and iOS.
Progressive web apps are designed to work on any browser that is compliant with the appropriate web standards. As with other cross-platform solutions, the goal is to help developers build cross-platform apps more easily than they would with native apps. According to Google Developers, the characteristics of a PWA are:
- Progressive — Works for every user, regardless of browser choice, using progressive enhancement principles.
- Responsive — Fits any form factor: desktop, mobile, tablet, or forms yet to emerge.
- Faster after initial loading - After the initial loading has finished, the same content and page elements do not have to be re-downloaded each time.
- Connectivity independent — Service workers allow offline uses, or on low quality networks.
- App-like — Feels like an app to the user with app-style interactions and navigation.
- Fresh — Always up-to-date due to the service worker update process.
- Safe — Served via HTTPS to prevent snooping and ensure content hasn't been tampered with.
- Discoverable — Identifiable as an “application” by manifest.json and service worker registration, and discoverable by search engines.
- Re-engageable — Ability to use push notifications to maintain engagement with the user.
- Installable — Provides homescreen icons without the use of an App Store.
- Linkable — Can easily be shared via a URL, and does not require complex installation.
The technical baseline criteria for a site to be considered a progressive web app by browsers were described by Russell in a follow-up post:
- Originate from a secure origin. Served over TLS and green padlock displays (no active mixed content).
- Load while offline (even if only a custom offline page). By implication, this means that progressive web apps require service workers.
- Reference a web app manifest with at least the four key properties: name, short_name, start_url, and display (with a value of standalone or fullscreen)
- An icon at least 144×144 large in PNG format.
- Use of vector graphics that are indefinitely scalable and require smaller file sizes.
Some progressive web apps use an architectural approach called the App Shell Model. In this model, service workers store the Basic User Interface or "shell" of the responsive web design web application in the browser's offline cache. This model allows for PWAs to maintain native-like use with or without web connectivity. This can improve loading time, by providing an initial static frame, a layout or architecture into which content can be loaded progressively as well as dynamically.
The web app manifest is a W3C specification defining a JSON-based manifest (usually labelled manifest.json) to provide developers a centralized place to put metadata associated with a web application including:
- The name of the web application
- Links to the web app icons or image objects
- The preferred URL to launch or open the web app
- The web app configuration data
- Default orientation of the web app
- The option to set the display mode, e.g. full screen
This metadata is crucial for an app to be added to a home screen or otherwise listed alongside native apps.
iOS Safari partially implements manifests, while most of the PWA metadata can be defined via Apple-specific extensions to the meta tags. These tags allow developers to enable full-screen display, define icons and splash screens, and specify a name for the application.
Progressive Web App execution contexts get unloaded whenever possible, so progressive web apps need to store majority of long-term internal state (user data, dynamically loaded application resources) in one of the following manners
Web Storage is a W3C standard API that enables key-value storage in modern browsers. The API consists of two objects, sessionStorage (that enables session-only storage that gets wiped upon browser session end) and localStorage (that enables storage that persists across sessions).
A service worker is a web worker that implements a programmable network proxy that can respond to web/HTTP requests of the main document. It is able to check the availability of a remote server and to cache content when that server is available, and serve that content later to the document. Service workers, like any other web workers, work separately from the main document context. Service workers can handle push notifications and synchronize data in the background, cache or retrieve resource requests, intercept network requests and receive centralized updates independently of the document that registered them, even when that document is not loaded.
Service workers go through a three-step lifecycle of Registration, Installation and Activation. Registration involves telling the browser the location of the service worker in preparation for installation. Installation occurs when there is no service worker installed in the browser for the webapp, or if there is an update to the service worker. Activation occurs when all of the PWAs pages are closed, so that there is no conflict between the previous version and the updated one. The lifecycle also helps maintain consistency when switching among versions of service worker since only a single service worker can be active for a domain.
Indexed Database APIEdit
Indexed Database API is a W3C standard database API available in all major browsers. The API is supported by modern browsers and enables storage of JSON objects and any structures representable as a string. Indexed Database API can be used with a wrapper library providing additional constructs around it.
Application Cache (or AppCache or HTML5 cache manifest) is an earlier technology that allowed the application to cache content in advance for later use when the device is offline. It works adequately for single-page applications which it was designed for, but fails in problematic ways for multi-page applications like wikis. As of May 2019, the technology is supported by major browsers and in use for years by some sites, but it is already deprecated in favor of service workers and will eventually be removed.
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